Interferometry includes techniques of superimposing two electromagnetic waves to extract information about one or both of the waves. These electromagnetic waves may be laser generated optical waves. Optical interferometer position measurement systems can provide very precise position and distance information for dimensional measurements and motion control. Optical heterodyne interferometers use an optical reference source wave having a frequency f1 mixed with a second optical wave having a frequency f2. The result of the mixing produces a summation frequency component f1+f2 and a difference frequency component f1−f2. The difference frequency component f1−f2 can be more easily measured for phase and frequency shifts than directly measuring the second optical wave.
One source of measurement error for optical heterodyne interferometers is periodic error of the optical reference source. Periodic error (also called polarization distortion and polarization error) results from an impure polarization state of the optical reference source wave. The solution to date has been to use waveplates to correct the polarization state and limit tolerances on the optical materials, coatings, and geometric errors on elements that the optical reference beam transverses. Polarization ellipticity can be corrected in one optical wave with waveplates but cannot generally be corrected in both optical waves simultaneously with waveplates. Also, non-orthogonality of the polarization states cannot be corrected by waveplates.
Therefore, there is a need to provide improved polarization and reduced periodic error for optical reference sources and optical heterodyne interferometers.